Solve the Modbus problem in front of you.
Practical notes built around the decision you need to make: which address to read, why a value is wrong, where a device is listening, or what evidence to keep.
- GUIDES
- 19
- FOCUS
- Frames · values · devices
Turn register words into values you can trust.
Resolve notation, offsets, data widths and word order before a plausible number becomes a bad engineering decision.
Understand zero-based PDU offsets, 40001 notation, address spaces, function codes, and the checks that prevent off-by-one Modbus reads.
→01.02FC03 vs FC04: holding registers and input registers are not the same tableUnderstand when to use Modbus FC03 or FC04, why the same numeric address can exist in both spaces, and how to verify a vendor register map.
→01.03Modbus Unit ID on RTU, TCP, and serial gatewaysUnderstand Modbus Unit IDs on RTU buses, direct TCP connections, and TCP-to-serial gateways without confusing the server address with the IP endpoint.
→01.04Decode 32-bit Modbus floats without guessing word orderA practical method for proving Modbus float32 word order, byte order, register width, scaling, and engineering units from raw words.
→Find the route without guessing the endpoint.
Start from an unknown serial bus or network and narrow the search without assuming the unit, port or readable range.
Plan a bounded Modbus register scan by proving transport and Unit ID first, estimating request count, and separating readability from meaning.
→02.02How to find Modbus TCP devices without assuming port 502Find Modbus TCP and TLS endpoints by separating live-host discovery, open TCP ports, valid Modbus responses, Unit IDs, and certificate failures.
→Separate wiring faults from protocol faults.
Use symptoms, frames and physical-layer checks to tell a timeout from a Modbus exception or a damaged RTU response.
Check serial ownership, framing, polarity, topology, bias, termination, Unit IDs, and request evidence before debugging the register map.
→03.02How to calculate a useful Modbus RTU timeoutEstimate Modbus RTU request and response wire time from baud rate and frame length, then add device turnaround and a measured safety margin.
→03.03Modbus exception, timeout, or CRC error: three different failuresDiagnose Modbus exceptions, timeouts, CRC failures, and transport errors by identifying which layer completed and what evidence to collect next.
→03.04Modbus exception codes: a practical troubleshooting referenceDecode Modbus exception codes 01, 02, 03, 04, 05, 06, 08, 0A, and 0B and decide what to check after each response.
→03.05Modbus TCP Security: TLS certificates, trust, and connection failuresTroubleshoot Modbus TCP Security by separating reachability, TLS handshake, X.509 identity, certificate trust, client authentication, and Modbus responses.
→03.06Safe Modbus writes with FC05, FC06, FC15, and FC16Plan and verify Modbus coil and register writes with explicit scope, raw-value checks, read-back, interlocks, and recoverable test conditions.
→Keep enough context to reproduce what happened.
Capture the frames, decoded values and project state another engineer needs to inspect the same failure later.
Start from a reviewed register map.
Apply a public device definition, verify it against the installed hardware and keep installation-specific settings in the project.
Configure an Eastron SDM630-Modbus V2 read using FC04, two-register IEEE 754 values, normal word order, and Busloom's public device definition.
→05.02How to turn a vendor register map into a reusable Busloom projectConvert a PDF or spreadsheet register map into address spaces, sources, groups, tags, data types, units, and verified Busloom views.
→Keep transport and function semantics explicit.
Compare RTU with TCP and trace the common read, write, diagnostic, identification, and exception function codes.
Compare Modbus TCP and Modbus RTU framing, addressing, timing, topology, diagnostics, gateways, TLS, and commissioning workflows.
→06.02Modbus function codesA practical reference for common Modbus read, write, diagnostic, identification, and exception function codes, including the exact address spaces they operate on.
→Follow one problem from route to evidence.
Use complete commissioning and troubleshooting sequences instead of treating each Modbus symptom as an isolated setting.
A practical workflow for finding a Modbus energy meter, proving addressing and float order, importing a register map, trending measurements, and keeping evidence.
→07.02Troubleshoot a Modbus TCP-to-RTU gatewaySeparate Ethernet, TCP, gateway routing, Unit ID, RTU framing, downstream timing, Modbus exceptions, and register decoding when a gateway route fails.
→Calculate first, then take the evidence to the bench.
Use the browser tools for addresses, register values, frames, and CRC, then continue with the detailed guides when you need the protocol context behind the result.